1. Field of The Invention
The present invention relates to a sheet shape metal material used for a fixing belt or the like in an image forming apparatus using toner and a fixing belt and a fixing apparatus using the same. More particularly, it concerns a metal sheet presenting a high fatigue resistance against repeated deformation and a fixing belt, a fixing apparatus, and further an evaluation of fatigue resistance of metal material for this purpose.
2. Description of Related Art
As an example of application where the metal sheet is subjected to a repeated deformation, a fixing belt in an image forming apparatus of belt fixing type can be cited. Namely, the fixing belt has an endless loop form, but curls in the opposite direction at the point of fixing nip, being pinched by two rollers. As the result, the respective portions of the fixing belt are repeatedly deformed by rotation. In general, conventionally, as the fixing belt material, electrocast nickel foil is often used as base material, coated with a releasing layer of silicone rubber or the like on one side.
However, in the typical fixing belt of the related art, the fatigue resistance against a repetitive deformation was not necessarily sufficient. Consequently, in a long use, it broke due to the metal fatigue, and could not meet the target life. Particularly, this tendency is evident for a higher paper advancing speed or for a wide size paper. To solve this, it has been devised to increase the base material hardness, or to adopt a two-layered structure with metals of different hardness. However, the former did not prove to be an effective measure, because the nip pressure had to be increased accordingly. The latter has increased the cost because of complicated manufacturing process, and presented the problem of the presence of many factors affecting quality variation.
The present invention has been achieved in view of the aforementioned problems of the related art. In short, it is an object of the present invention to provide a metal sheet having a high fatigue resistance against repeated deformation and a fixing belt and a fixing apparatus using the same. Also, it is another object of the invention to provide an evaluation apparatus of fatigue resistance of metal material and a method thereof.
A flexible metal sheet with high fatigue resistance of the present invention devised in order to solve this problem is a flexible sheet member satisfying at least one of the following three expressions:
0.7 less than IH/ILxe2x89xa61xe2x80x83xe2x80x83(1)
0.25 less than ZL/ZHxe2x89xa61xe2x80x83xe2x80x83(2)
0.7 less than QH/QLxe2x89xa61xe2x80x83xe2x80x83(3)
for effective value of current density, impedance and calorific power, respectively when two alternating voltages of the same effective voltage value and different in frequency, namely a low frequency first alternating voltage and a high frequency second alternating voltage are applied. It is needless to say that it is better that all three expressions are satisfied. Here, I, Z, and Q represent effective value of current density, impedance and calorific power, respectively. Suffixes H and L represent high frequency (second alternating voltage) application and low frequency (first alternating voltage) application, respectively.
The inventor has studied diligently to find an evident correlation between the voltage-current characteristics against high frequency and the fatigue resistance of metal material. That is, a metal material showing voltage-current characteristics against a high frequency comparable to that against a low frequency tends to show a higher fatigue resistance. On the contrary, a metal material showing voltage-current characteristics against a high frequency inferior to that against a low frequency tends to show a lower fatigue resistance. Therefore, the fatigue resistance level of metal material can be judged by comparing voltage-current characteristics against high frequency and low frequency. Further, if at least one of the aforementioned three relations is satisfied, it can be said to be excellent as high fatigue resistance metal sheet. Note that in this Application, xe2x80x9cmetalxe2x80x9d includes alloyed metals. A representative example of high fatigue resistance metal sheet of the present invention is an electrocast nickel of 100 xcexcm or less in thickness, manufactured under predetermined electrocasting conditions.
As for the reason of such correlation between the high frequency characteristics and the fatigue resistance, the inventor presumes as follows. Namely, bad frequency characteristics mean that the mobility of conduction electrons in the metal crystal is low under the high frequency. This is supposed to be provoked by the presence of many factors impeding the movement of conduction electrons, such as many lattice defects or impurities, or irregular crystalline granularity. These factors are also factors impeding movement of displacement during the deformation, and they are also supposed to act as starting points of cracking. Therefore, those bad in high frequency characteristics are low in fatigue resistance, and those good in high frequency characteristics are also good in fatigue resistance. Besides, the alternating voltage may be direct current on and off, an alternating current (whatever the waveform may be) is preferable, if it is possible. For the alternating current, as the polarity changes periodically, the crystalline quality is reflected on the high frequency characteristics more appropriately.
In addition, the fixing belt of the present invention includes an endless belt shape metal sheet and a releasing layer formed on the outer surface thereof, wherein the metal sheet satisfies at least one of the aforementioned relations. Moreover, the fixing apparatus of the present invention comprises a first roller, a second roller, a fixing belt including an endless belt shape metal sheet and a releasing layer formed on the outer surface thereof, and that is wound around the first roller and the second roller, and a third roller disposed in a way to be pressed against the first roller via the fixing belt, wherein the metal sheet satisfies at least one of the aforementioned relations.
Also, the evaluation apparatus of metal material fatigue resistance according to the present invention comprises a high frequency power source for applying an alternating voltage to a test piece, a current measuring instrument for measuring current through a test piece, and a controller for calculating the ratio of currents through a test piece when a plurality of alternating voltages of the same effective voltage value and different in frequency are applied to a test piece from the high frequency power source, and evaluating fatigue resistance of the test piece based on the calculation results thereof.
Moreover, the evaluation method of metal material fatigue resistance according to the present invention comprises the steps of: applying a plurality of alternating voltages of the same effective voltage value and different in frequency to the test piece and measuring the current through the test piece when each alternating voltage is applied; calculating the ratio of these currents; and evaluating fatigue resistance of the test piece based on the calculation results thereof.
In this evaluation apparatus and evaluation method, the fatigue resistance of the test piece as a whole is evaluated by comparing currents through the test piece in case of high frequency and that in case of low frequency. To be more specific, it is evaluated that the closer the current ratio is to 1 in the case of high frequency and in the case of low frequency, the higher is fatigue resistance of the test piece. Obviously, the impedance may be compared considering not only the current value, but also phase components.
Otherwise, the evaluation apparatus of metal material fatigue resistance according to another aspect the present invention comprises a high frequency power source for applying an alternating voltage to a test piece, a calorific power measuring instrument for measuring the calorific power at each point of a test piece, and a controller for comparing the calorific power of the same point of a test piece when a plurality of alternating voltages of the same effective voltage value and different in frequency are applied to a test piece from the high frequency power source, and evaluating fatigue resistance of the test piece based on the comparison results thereof.
Similarly, the evaluation method of metal material fatigue resistance according to another aspect of the present invention comprises the steps of: applying a plurality of alternating voltages of the same effective voltage value and different in frequency to the test piece and measuring the calorific power of the same point of the test piece when each alternating voltage is applied; comparing the measured calorific power; and evaluating the test piece fatigue resistance based on the comparison results.
In these cases, it is evaluated that the closer the calorific powers in the case of high frequency and in the case of low frequency are to each other, the higher is fatigue resistance at that point of the test piece. In these cases, further, not only the evaluation of the test piece as a whole but also the evaluation of a particular point is possible. Moreover, a two-dimensional evaluation of fatigue resistance in the test piece is also possible by performing the evaluation for a plurality of points.
Thus, according to the present invention, a metal sheet having a high fatigue resistance against repeated deformation and a fixing belt and a fixing apparatus using the same are provided. In addition, there are provided an evaluation apparatus of metal material fatigue resistance and a method thereof.